Due to their well known lower production cost, longer life and greater reliability, solid state amplifier circuits have replaced vacuum tube circuits in almost every application. In the area of RF amplifiers with high power output, however, vacuum tube circuits still have such a large cost advantage that they are preferred in most situations. Only where there is a great premium on long-term maintenance is it considered economical to use solid state devices. There are two major reasons for this.
First, suitable semiconductor devices for this application are expensive. A number of companies market families of RF metal oxide, field effect transistors (MOSFETs) designed for this service and known as RF power MOSFETs. Examples of such devices are the Motorola MRF series (such as the Motorola MRF 150), the SGS Thompson SD series and the Philips BLF series. The Direct Energy DE series (such as the DE 275) use high voltage switching MOSFET devices in expensive RF packages. Each of these currently sells, in quantity, at about fifty dollars or more for a 150 watt device. The high cost is due, at least in part, to the fact that to deliver high power at RF, special packaging is required. See, for example, U.S. Pat. No. 4,891,686.
As shown in FIGS. 1-3 each of the specially packaged RF power MOSFETs has flat or ribbon-like leads providing lead inductances between about 1 nH and 4 nH. FIGS. 1 and 2, for example show the packaged DE-275, while FIG. 3 shows the Motorola MRF150.
Second, for high power outputs, i.e., outputs greater than 500 watts, the output of several devices must be combined. Straight parallel combining, however, reduces the output impedance of the combination still further. In fact, it divides the impedance of a single device by the number of devices in parallel. A typical RF MOSFET (such as Motorola MRF150) has a optimum load impedance of approximately 3-4 ohms. Thus, if each device is operating into 3.2 ohms, two devices in parallel operate into 1.6 ohms. The impedance of a 50 ohm load, therefore, must be transformed to match the combination of devices. This requires the impedance transforming capability of hybrid combiners. Because of the high currents needed to generate high power at these low impedances, the needed combiners are also very expensive. Further, because hybrid combiners typically combine only two paths into one, to combine the power output of eight MOSFETs, for example, seven combiners are required. The added expenses, therefore, of very low inductance device packaging and many high current combiners make the current solid state, high power, RF amplifier non competitive in many applications.
This invention is based on the utilization of non-RF switching MOSFETs, on the market for over four years, but prior to the present invention thought to be unsuitable for RF amplifiers because of their high lead inductance. According to the present invention, they are used in a way that eliminates the need for expensive packaging and combiners. There has been at least one suggestion that an amplifier can be constructed with non-RF MOSFETs. In lkeda, "Development of a Solid State Radio Transmitter with MOS/FET", published in the IEEE Transactions on Broadcasting, Vol. BC-26, No. 4, (December 1980), pp 99-112, it has been suggested that the JEDEC TO-3 packaged MOSFET can be used in an RF power amplifier for use in transmitting in the AM bandwidth (generally between about 0.5 and 1.5 MHz. Ikeda has described RF amplifiers operating as high as 9.535 MHz, where efficiency drops to 70%. It is clear that greater frequencies can not be efficiently achieved, particularly at 65 MHz and higher where certain applications (such as magnetic resonance imaging) require operation of an amplifier at such RF frequencies. Further, the TO-3 packaged device provides relatively high lead inductances of between about 12 nH and 25 nH, and is not configured for modern production techniques where the leads are made coplanar to one another. As shown in FIGS. 4 and 5, the device is enclosed within a metal casing with two wire leads extending from the bottom of the device.
At the same time that amplifier oriented semi-conductor designers have been creating the RF power MOSFET by lowering the inductances of such devices, switching oriented designers have been improving the power switching MOSFET for better switching performance by increasing its breakdown voltage and switching speed. The result is a class of switching devices that have ordinary, inexpensive packaging, but have high breakdown voltage and enough speed to operate efficiently high into the RF range. This class may properly be called high voltage, power switching MOSFETs, as opposed to and distinguishable from RF power MOSFETs. Examples of such devices include non-RF devices, such as the APT 5085 packaged as the TO-247. Other devices are available and packaged as the TO-220. A non-RF MOSFET device packaged in accordance with the JEDEC TO-247 format is shown in FIGS. 6 and 7. In general the package is provided with a MOSFET encased with leads co-planar and extending from one side of the casing. As in the case of the TO-3 device the lead inductances of these non-RF devices are significantly higher than the 1-4 nH of the special RF packages.